Typically, the conventional IR detectors are of a type requiring cooling of the light detection element (detecting IR radiation). The conventional IR detectors, for example Focal Plane Array (FPA) detectors, are usually cooled to a cryogenic temperature and are typically associated with (e.g. enclosed within) cryogenically cooled Dewar. The latter includes a cold shield and a cold filter of the Dewar, and has a Dewar enclosure including a warm optical window as a part thereof. The detector is placed in a housing and is located behind the cold shield and cold filter. The cooling mechanism (which typically cools also the cold shield and the cold filter) supports the increase of the signal-to-noise ratio of the IR radiation detection by reducing a thermal noise (in the IR spectral range), namely noise associated with thermal emission from the detector housing which is disposed to environment. The cold shield is typically configured for reducing the thermal noise from the detected signal by minimizing the IR radiation that arrives to the detector from regions out of the field of view of the complete system.
Such cooled IR detectors typically utilize an IR radiation-sensitive detection module (e.g. FPA detector), a cold shield and a cold filter. FIG. 1 exemplifies a conventional detector Dewar assembly 10. As shown, the assembly 10 includes a light sensitive element (detector) 12, enclosed inside a Dewar (housing) 14. The housing 14 has an optical window 16, collecting IR radiation to be sensed by detector 12. The optical window 16 is thus a part of the Dewar enclosure. The housing 14 with the window is exposed to environment, the optical window is thus called “warm window”. The housing contains a cold shield 18 surrounding the detector element 12 and being thermally isolated from inner surface of the enclosure 14 by vacuum and by a low emissivity coating of the outer surface of the shield 18. The cold shield carries a cold filter 20. The detector element 12 is coupled to an internal cryogenic cooler 22.
Various techniques have been developed for reducing the thermal noise in IR detection systems. For example, U.S. Pat. No. 4,820,923 describes an uncooled reflective shield for cryogenically cooled radiation detectors. Here, a warm shield reflector is used with a cryogenically cooled radiation detector. The warm shield has a reflective surface of toroidal shape. The surface has geometric properties which cause a ray emanating from the detector to be reflected such that a ray is imaged as a defocused ring outside of and surrounding the active detector area. Several such segments are located in front of a small, cryogenically cooled detector shield, to provide an overall detector shielding effect similar to that of a larger, cryogenically cooled shield.
U.S. Pat. No. 6,969,840 discloses an all-reflective telescope which has, in order, a positive-optical-power primary mirror, a negative-optical-power secondary mirror, a positive-optical-power tertiary mirror, a negative-optical-power quaternary mirror, and a positive-optical-power field lens. The mirrors and lens are axisymmetric about a beam axis. The light beam is incident upon an infrared detector after reflecting from the quaternary mirror. A cooling housing encloses the detector and the field lens, but does not enclose any of the mirrors. An uncooled warm-stop structure outside of the cooling housing but in a field of view of the detector is formed as a plurality of facets with reflective surfaces oriented to reflect a view of an interior of the cooling housing back to the interior of the cooling housing.
U.S. Pat. No. 7,180,067 discloses an infrared imaging system using an uncooled elliptical surface section between reflective surfaces to allow a detector to perceive a cold interior of a vacuum chamber rather than a warmer surface of a structure or housing. In this way, background infrared radiation from within the system may be minimized.